All gases, including air, contain moisture. When a gas is compressed to the point at which its moisture content can no longer remain in a vapor state, it precipitates as a liquid. This is a very serious problem in any compressed gas operation, and as a practical matter, compressed air must be kept as dry as possible in order for it to be of any commercial value. For example, water interferes with air operated equipment by rusting components, mixing with any oil present to form sludge and varnish, and freezing the equipment in cold weather. This results in the delivery of less power than would otherwise be available, since water adds to the mass of the air flow and, therefore, to the friction within the delivery system itself. Furthermore, because water is incompressible, it cannot contribute to the production of power, for instance in air tools.
Water also causes damage by virtue of its accumulation in the compressed gas receivers with which virtually all compressors are associated. Air receivers are required since they act as a storage vessel for the compressed gas, so that the compressing equipment will not over-heat due to continuous operation. In addition, the compressor motor and pump do not have to cycle as much each time there is a demand for air, inasmuch as air is already stored in the receiver, ready for use. The latter factor is important since frequent start-up of the compressor is a major cause of motor failure. An air receiver also eliminates the pulsations generated by the compression equipment, allowing smoother power delivery.
Once the air is stored in the receiver, however, it has a chance to cool, permitting water separation to occur in the receiver itself as the moisture precipitates during cooling. This produces denser air, allowing more power to be delivered to the application; however, the water must be removed for reasons which include those previously described. It is the removal of such water and associated waste to which the invention addresses itself.
When acquisition of a compressed air system is under consideration, the pump, motor and receiver tank sizing are given careful thought. Such sizing is important because a 3 horsepower compressor cannot be used when more compressing capacity is needed. Likewise, the use of a 20 horsepower compressor when a 10 horsepower unit would be adequate is wasteful. Consequently, all of the demands on the compressed air system are taken into consideration at the time of system design, and the receiver size is usually matched to the compressor's capacity to produce air, the latter, in turn, being matched to the rate at which compressed air is required by the application.
While receiver size is usually dictated by the horsepower of the compressor, it's size may be varied somewhat, and there are general sizing guidelines commonly used in the industry. For instance, an 80 gallon tank is considered standard for a 5 horsepower air compressor, and while a smaller or larger receiver is sometimes used with such a compressor, experience has shown that a tank of that size strikes an optimum compromise between size, cost, and the amount of energy involved in filling it. It is understood, however, that if a larger storage vessel is needed, a larger compressor is required to handle the application.
Air storage requirements, the need to limit compressor cycling, and the provision of sufficient storage space to allow the air to cool before use all influence receiver sizing; consequently, different circumstances dictate the use of different receivers. Generally speaking, larger receivers are used with larger horsepower compressors, since the amount of air flowing through them is so great that smaller receivers would not accomodate influencing factors such as those described in the preceding.
While it is advantageous that the air in the receiver cool, and that the water separate from it, the accumulation of water in the receiver must be avoided. For example, when the water accumulates, it takes up space in the tank, effectively reducing the capacity of the receiver. Since water cannot be compressed, an 80 gallon tank half full of water has only 40 gallons of space left for air storage. It is apparent, therefore, that if the receiver capacity has been halved, and that it takes only half as much time to empty, and half as much time to fill. This means that the compressor has to cycle on and off twice as often as would otherwise be the case, and that and it pumps only half as long during each cycle. It is this "rapid cycling" that destroys compressor motors since the inertial start-up loads are so extremely large. The armature in larger integral horse power motors may, for instance, weigh as much as 100 pounds by itself, and the motor must also turn the compressor pump from a standing start, all of which creates a severe strain on the motor. If repeated too often, the strain causes the motor to wear out quickly, and it is, as stated, one of the problems associated with water accumulation inside air receivers.
A further problem encountered when the receiver capacity has been reduced by water is that it takes proportionally less time for the air in the receiver to be consumed because there is less air stored to begin with. The air in the receiver also has less time to cool, and as a consequence, air leaving the tank is warmer, less dense, and wetter, reducing the amount of air power delivered to the equipment, and increasing the amount of water entering it. The result is the rapid destruction of air operated tools and equipment. For all the preceding reasons, therefore, it is important to assure that the receiver be kept as clean as possible.
While all receivers have provision for bottom drainage, it is difficult to find a valve suitable for the purpose. In this regard, receiver tank drainage provides a severe test for any drain valve, due to the fact that the inside of the tank corrodes as a consequence of water condensing on its sides, resulting in the distribution of particulate scale matter over its bottom surface. At the same time, oil from the compressor intermixes with scale from the compressor head, and from the pipes leading to the receiver, forming a mixture that combines with the water and scale present on the inside of the tank to form an acidic sludge which corrodes and plugs conventional drain valves, tending to render them inoperative. When allowed to dry, the sludge, particulate matter, etc., forms a hard plaque not unlike cement, providing a difficult task for any valve to accomodate.
The valve of the invention is designed to handle this difficult application, however, and all features of the valve, from the type of materials used in its construction, to the unique dished-out face, are intended for use where the material to be drained is not clean water, but rather is a combination of substances, including particulate matter, and where the pressurized integrity of the vessel must be maintained, as in the case of the receivers referred to.